286 H. R. Chaplin 
1 C 
PF. = D) pV ih 
L 
ApS = ky 5 pV2S 
where kp is the effective pressure recovery factor. This gives 
M= SL/L aeleeieaeae 
2 fo Pe Si) 2h) |) Res 
Open-jet wind tunnels typically have energy ratios around 3.0. Using this value, and 
assigning (arbitrarily, and probably optimistically) the value of 0.7 to k, would give 
- S 
M2 0.6 Te 
as a rough estimate of actual hovering performance, under favorable conditions. This is the 
same as the estimated actual hovering performance of well-designed air curtain vehicles. 
This numerical result is questionable, due to the assumptions employed. Of greater 
significance is the demonstration that, for the diffuser-récirculation concept, just as for all 
of the other ground cushion concepts, the performance is directly dependent on the size/ 
height ratio, S/AC. 
More detailed information on the diffuser-recirculation concept is found in Ref. 12. 
SIMPLIFIED ENGINEERING ANALYSIS OF THE AIR CURTAIN VEHICLE 
The foregoing “simplified ideal theory” analyses serve only to give mathematical expression 
to the basic ideas involved in the various ground cushion concepts. Comparisons between 
the various concepts on this basis are still only comparisons of ideas. Realistic compari- 
sons of engineering merit will require not only more complete analysis, but a considerable 
background of systematic empirical information. Such comparisons are not possible at the 
present state of the art. More complete analyses of several of the concepts will be found in 
the references, but only in the case of the air curtain concepts is there a sufficient body of 
systematic experimental data to support a realistic assessment of the practical performance. 
It would be superfluous to repeat here any detailed analysis of air curtain vehicle per- 
formance. It may be useful, however, to follow a simplified development, along the lines of 
the simplified ideal theory, but accounting, in an elementary way, for the most significant of 
the effects previously neglected. These are: 
1. Internal losses, accounted for by a duct efficiency 74, compressor efficiency 7p, and 
internal efficiency 7,_, = Np Na: 
2. Base pressure loss, accounted for by an augmentation efficiency 7, . 
3. Aerodynamic lift coefficient Cz), and parasite drag coefficient Cpz, produced by 
the external flow field. 
